Why not Night Slash? BB + it has nearly great coverage, and in CB set it would be an option over the Dragon-type attacks of the last slot.
Just guessing.
A super effective Night Slash is depressingly weak when you consider Haxorus's access to ALREADY almost-perfect coverage in a Dragon-type STAB and Earthquake. Better yet, Mold Breaker alleviates almost all concerns concerning coverage, and Night Slash barely fills in the holes. If the secondary Dragon-type move is replaced, Haxorus no longer has a reliable cleanup move as well.
